The present invention relates to a studless tire, more particularly to a tread structure capable of improving uneven wear resistance.
In general, studless tires are provided in the tread portion with blocks and a large number of sipes to improve snow performance and ice performance.
In case of a studless tire provided with circumferential ribs in addition to blocks as shown in FIGS. 6A and 7A, edges of the blocks which edges are adjacent to the ribs with sipes (s) sometimes wear unevenly as shown in FIGS. 6B and 7B as hatched part.
It is therefore, an object of the present invention to provide a studless tire in which the above-mentioned uneven wear can be effectively controlled without sacrificing ice performance, snow performance and the like.
According to one aspect of the present invention, a studless tire comprises a tread provided with blocks and at least one rib provided with sipes each extending across the overall width of the rib,
the sipes comprising first sipes and second sipes alternately arranged in the longitudinal direction of the rib at circumferential pitches in a range of from 0.028 to 0.04 times a ground contacting width of the tread,
each of the first sipes and second sipes comprising a deep central part, a shallow part on one side thereof and a mid-depth part on the other side thereof,
the deep central part having a depth in a range of from 0.70 to 0.80 times a depth of a main longitudinal groove defining the rib, the mid-depth part having a depth being in a range of from 0.40 to 0.60 times the depth of the main longitudinal groove, and the shallow part having a depth in a range of from 0.04 to 0.35 times the depth of the main longitudinal groove,
the shallow part and mid-depth part of each first sipe being positionally reverse to those of the second sipes so that, on each side of the rib, the shallow parts alternate with the mid-depth parts.
Therefore, even if the sipes are arranged at small pitches, circumferential and axial deformation of the rib during running is controlled. As a result, uneven wear of the adjacent blocks"" edges can be effectively reduced while receiving improved edge effect from the sipes.
Here, the xe2x80x9csipexe2x80x9d means a narrow slit or cut having a width of less than 1.5 mm. The ground contacting width means the width between the tread edges or axial outermost edges of the ground contacting region under such a condition that the tire is mounted on its standard rim and inflated to its standard pressure, and then loaded with a standard load. The standard wheel rim is a wheel rim officially approved for the tire by standardization association, organization and the like, such as JATMA (Japan and Asia), TandRA (North America), ETRTO (Europe), STRO (Scandinavia) and the like. The standard pressure is the xe2x80x9cmaximum air pressurexe2x80x9d in JATMA, the xe2x80x9cInflation Pressurexe2x80x9d in ETRTO, the maximum pressure given in the xe2x80x9cTire Load Limits at Various Cold Inflation Pressuresxe2x80x9d table in TandRA or the like. In case of passenger car tires, however, 180 kPa is used as the standard pressure. The standard load is the xe2x80x9cmaximum load capacityxe2x80x9d in JATMA, the xe2x80x9cLoad Capacityxe2x80x9d in ETRTO, the maximum value given in the above-mentioned table in TandRA or the like.